Methods for industrial gas and vapor analysis have evolved along two separate paths. The first involves complex instruments for example infrared spectroscopy, gas chromatography, and mass spectrometry. The development of microcontrollers and microcomputers has led to smaller and more rugged versions of these analytical instruments that were once only found in laboratories. These instruments are very powerful, but they have disadvantages. They are costly and maintenance-intensive. They are usually located far from the gas or vapor source in climate controlled enclosures. The gas or vapor sample is usually transported to the analyzer in a heated tube. This process delay will not provide real time information. These instruments are not likely to be affordable candidates for real-time, in-situ applications such as combustion gas analysis or chemical process analysis. The second evolutionary path is the development of emerging chemical sensor technology.
There are many industrial applications for gas and vapor sensor technology. For example, there is the detection of hazardous gas in the workplace for worker safety, the analysis of products in the combustion of fuel for better control of the air-fuel mixture and the analysis of feed and product streams for the optimization of product yield and waste reduction.
The use of sensors in workplace gas detection instruments is routine. The technologies used are individual sensors which are installed to detect a single gas or vapor whose concentration, in the local environment, may approach a hazardous level. The examples of these gases include hydrogen sulfide, carbon monoxide, chlorine, ammonia, hydrogen, methane and many others.
The development of sensors is ongoing for use in aerospace applications where weight and size are critical issues. For example, the magnitude and location of hydrogen leaks in liquid fueled rockets is important in launch readiness applications.
The use of gas sensors in combustion analysis to control fuel air mixtures began with the use of a solid state electrochemical sensor for oxygen. The sensor was designed to detect the precise air-fuel mixture in automobiles that would not be too rich or too lean. Additional sensors to analyze the mixture for carbon monoxide and oxides of nitrogen are important in fine tuning the mixture.
The use of chemical sensors in chemical process streams has the potential of improving process efficiency and the resulting yield. A direct result of process improvement is the potential to reduce the amount of waste. For example, the water concentration in a silicone process feed stream is an important parameter in the final product quality.
The degree to which these sensors can uniquely identify the gas or vapor they were designed to detect depends on the sensitivity of the sensor to other interfering species and the concentration of those species. The ability to resolve the target gas or vapor is called the selectivity. There are very few examples of sensors which are highly selective (e.g. greater than a ten fold difference in relative sensitivity), and even these do not work ideally. For example, a sensor whose target gas will give an incorrect indication of the target gas when the concentration of the interferant is high enough. In reality, this limitation is relieved by using the sensor in a situation where the situation described above is unlikely.
An additional way to improve the quality of the information provided by a sensor is to process information from an array of sensors.
One application for chemical sensors is in the detection of particular gases in mineral oil-filled electrical transformers.
Faults such as arcing, corona discharge, low energy sparking, severely overloading, pump motor failure and overheating in the insulation system can generate hydrogen (H.sub.2), acetylene (C.sub.2 H.sub.2), ethylene (C.sub.2 H.sub.4), and carbon monoxide (CO) in oil-filled electrical transformers. These conditions can result in transformer malfunction and may eventually lead to failure if not corrected. The detection of the presence of these four transformer fault gases is frequently the first available indication of a possible malfunction. There is a statistical correlation between transformer malfunction conditions and the fault gases they generate. This correlation has enable the evaluation of possible fault types with which by a key gas method.
The significant gases (key gases) and the four general fault types with which they are associated are as follows:
a.) Thermal-Oil--C.sub.2 H.sub.4, PA1 b.) Thermal-Cellulose--CO.sub.2, CO PA1 c.) Electrical Corona--H.sub.2 PA1 d.) Electrical-Arcing--H.sub.2, C.sub.2 H.sub.2
Because of this correlation, the possible type of fault in a transformer can be evaluated by the analysis of the gases generated in that transformer.
The utility industry has developed various diagnostic theories that employ ratios of certain key gases. The existence of such "gas ratios" makes it possible to improve the reliability of transformers. However, in order to evaluate the possible fault types using the various gas ratios (e.g. Rogers Ratio Method, Doernenburg Ratio), the gases must be distinguished and analyzed.
Currently gas chromatography is used to identify and quantify gases dissolved in oil having a viscosity of 20 centistokes or less at 40.degree. C. (104.degree. F.). A significant disadvantage, to the user, is the lack of real-time analytical information.
For gas chromatography to be effective, the quantity and composition of gases dissolved in oil samples must remain unchanged during transport to the laboratory. Sampling necessarily involves the risk of exposing the oil to air and the concomitant loss of dissolved gases.
An alternative would be a chemical sensor immersed directly into the oil environment of the electrical transformer, operating on a real-time basis to indicate the presence of an incipient fault, and which would not require sampling the oil and subsequent analysis in a laboratory.
Selective sensors capable of distinguishing and analyzing particular components would exhibit a sensitivity (output/concentration unit) to a particular component that is different in magnitude than the sensitivity to a different component giving a signal.
The sensor response in a mixture of two components should ideally be a simple sum of the response (i.e. voltage) in the individual components. In this linear case, the effects can be resolved conventionally with an analytical algorithm such as one based on matrix algebra as taught by L. W. Potts for chemical analysis in Quantitative Analysis: Theory and Practice, Harper and Row, New York 1987, Chapter 13.
Chemically sensitive field-effect transistors (CHEMFETs) have been developed for the detection of specific compounds in liquid and gaseous environments, such as the ion sensitive CHEMFETs disclosed in U.S. Pat. No. 4,020,830 to Johnson, et al. and U.S. Pat. No. 4,305,802 to Koshiishi.
Other CHEMFETs have been produced that measure the concentrations of components in a gaseous state, as for example the devices disclosed in U.S. Pat. No. 3,719,564 to Lilly, Jr., et al. and described by Shimada, et al. in U.S. Pat. Nos. 4,218,298 and 4,354,308, and the suspended gate field-effect transistors (SGFETs) described by Jiri Janata in U.S. Pat. Nos. 4,411,741 and 4,514,263. These devices are relatively complex and costly to manufacture due to their multiple junctions and diffusion regions.
CHEMFETs in general, and SGFETs in particular are also not well suited to detection of combination of specific compounds or specific compounds in the presence of other potentially interfering chemical species. Combinations of discrete SGFETs with sensitivities to different compounds have been proposed in efforts to address such deficiencies. For example, in U.S. Pat. No. 4,368,480 to Senturia multiplexed CHEMFETs provide logic elements with varying on-off duty cycles. Nevertheless, such combinations are even more difficult and costly to manufacture than individual sensors.
A device and method for detection of fluid concentration utilizing charge storage in a MIS diode is disclosed in U.S. Pat. No. 4,947,104 to Pyke. MIS sensors detect and amplify the change in the work function of its metal electrode when gas adsorbs on its surface. A MIS sensor for hydrogen was described with a palladium metal electrode by M. S. Shiveraman et al., Electron Lett., 12,483 (1976) and Z. Li et al., IEEE/IEDM-85,125 (1985). MIS sensors with Ni and Pt electrodes for methane and carbon monoxide have been reported by T. L. Poteat, et al. J. Electron. Matl., 12,181 (1983).
The MIS tunnel junction was developed with a palladium electrode for hydrogen detection as reported by R. C. Hughes et al., J. Appl. Phys. 62(3), Aug. 1, 1987 (pp 1074-1083). Operated in reverse bias with an oxide layer thin enough for a measurable reverse current, this sensor is much more sensitive to hydrogen. In this example, the work function change is due to a layer of Pd-H dipoles at the metal insulator interface and due to a change in the composition of the metal on adsorbing hydrogen.
It is therefore desirable to provide methods and devices for detecting and analyzing specific gases in a mixture. It is further desirable to provide methods and devices to indicate the presence of the key fault gases in mineral oil, predict their generation rate, and alert the substation operator accordingly.